1. Field of the Invention
The present invention relates to a black and white photothermographic material. More particularly, the invention relates to a black and white photothermographic material which exhibits excellent image tone, excellent raw stock storage stability, and excellent image storage stability.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. For this reason, technology regarding thermal developing image recording materials for medical diagnosis and for graphic arts, which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, is required. The thermal developing image recording materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
Thermal image forming systems utilizing organic silver salts are described, for example, in the specifications of U.S. Pat. Nos. 3,152,904 and 3,457,075 and in “Thermally Processed Silver Systems” by D. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Walworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. In particular, photothermographic materials generally have an image forming layer in which a photosensitive compound (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region.
The photothermographic materials utilizing an organic silver salt have an advantageous characteristic of containing all components necessary for image formation in the film in advance and being capable of forming images only by heating. However, on the other hand, the photothermographic material has a problem in that it is difficult to attain high sensitivity due to generation of fog. In addition, the photothermographic material has a problem relating to storage stability in which, for example, sensitivity changes or fog increases during storage thereof. Moreover, because photosensitive silver halide grains remain in the material after image formation, there are serious problems in that film turbidity becomes high due to light absorption and light scattering, and fog increases during placement of the images under light conditions, which is called print-out.
As a reducing agent in the photothermographic material utilizing an organic silver salt, an o-bisphenol derivative is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2001-92075.
On the other hand, JP-A Nos. 2001-312026, 2003-215767, and 2003-215764, and U.S. Pat. No. 6,242,166 disclose photothermographic materials containing a color developing agent and a coupler. These materials use photosensitive silver halides such as silver chloride, silver bromide, silver chlorobromide, silver iodobromide, or silver iodochlorobromide. Because light scattering and light absorption due to the silver halide increase turbidity and opacity of the film, fogging becomes extremely high and is as high as 0.58 to 1.2 as described in the Examples of the above specifications. Accordingly, as described in JP-A Nos. 2003-215767 and 2003-215764, the obtained image is a primary image and is not an image for being directly viewed, and accordingly, the image is digitalized, and image processing is performed to reduce fogging and adjust gradation and color tone, whereby it is attempted to form a reprocessed image which can be provided for viewing.
The use of sulfonamido phenols as color developing agents is known. For example, JP-A Nos. 2001-330923, 2001-330925, and 2002-49123 disclose the use of a dye formed by a process using a coupling reaction of an oxidation product of sulfonamido phenols with a coupler, in order to improve image tone of a black and white photothermographic material. However, the use of conventional reducing agents and couplers cannot provide an image with desired color density and favorable color tone. There exists a problem in that yield of dye becomes low, because the reaction of forming a dye from a coupler and a developing agent is prevented by other various reactions that occur simultaneously at the time of development. For this reason, the conventional technology is insufficient to obtain desired color-forming dye density. Increasing the amount of coupler or developing agent for use in order to gain dye density is not only ineffective, but also does not satisfy photographic performance under the present circumstances because of occurrence of deterioration in storability.